A membrane switch comprises two base films lying on either side of a spacer, with contact points (electrodes) corresponding to each of the opposing surfaces. Switching action between conduction and insulation can be easily accomplished by pressing the base films, i.e. varying the spacing between the base films. Membrane switches have recently come into wide use for the keypads of cellular phones or portable personal computers or the various control panel switches of household electrical appliances such as VTRs and microwave ovens.
Because the switching action of a membrane switch involves repeated pressing, the base films used therefor must be flexible and resistant to deformation. Base films used for conventional membrane switches have generally been polyethylene terephthalate (hereinafter also abbreviated as “PET”) films because of their deformation resistance, adhesion with electrodes, bonding with printing pastes, and other properties.
Recently, however, membrane switches and remote control switches are becoming more common in the operating panels of automobile-related parts (car audio systems, car air conditioners, car navigation systems and the like). Base films of membrane switches have therefore been required to have deformation resistance at high temperatures that cannot be withstood by PET films. For example, vehicle interior temperatures are known to reach approximately 80° C. during the daytime in summer, and such interior temperatures often exceed the glass transition point (Tg) of PET. When a PET film is used as a membrane switch base film in such an environment, the high temperature burden results in deformation of the PET film which remains even after the burden no longer exists, and such warping of the PET film is a cause of switch malfunction.
As a means of improving this situation, Japanese Examined Patent Publication HEI No. 4-75610 has proposed using as the base films polyethylene naphthalenedicarboxylate (hereinafter also abbreviated as “PEN”) films which have a higher glass transition point than PET films. Specifically, Japanese Examined Patent Publication HEI No. 4-75610 discloses a membrane switch having contact points corresponding to each of the opposing surfaces of two base films, which employs as at least one of the base films a biaxial oriented polyethylene naphthalenedicarboxylate film having an F-5 value (5% elongation stress) of ≧11 kg/mm2, a density of ≦1.375 g/cm3 and a thermal shrinkage factor of ≦1.0% when heated at 120° C. for 30 minutes.
Also, Japanese Examined Patent Publication HEI No. 6-4276 discloses polyester film for a membrane switch composed of polyethylene naphthalate having a haze increase rate of no greater than 20% as defined by the following formula upon heat treatment at 150° C. for 2 hours:Haze increase rate={(H2−H1)/H1}×100 (%)(where H1 is the haze value before heat treatment and H2 is the haze value after heat treatment),and a thermal shrinkage factor of no greater than 0.5% in both the film processing direction (the direction of propagation of the film during continuous film processing, also referred to as the film longitudinal direction, machine direction, continuous film formation direction or MD direction) and the widthwise direction (the direction orthogonal to the film forming direction within the plane of the film, also referred to as the transverse direction or TD).